1. Field of the Invention
The present invention relates in general to transmission line structures that are formed from parallel suspended beams and are configured for lateral movement with respect to one another to effect implementation of a tunable phase shifter or a switch.
2. Description of the Background Art
The application of micro-electro-mechanical system (MEMS) technology to realize microwave devices has rapidly matured over the last several years. The technology has found some of its most promising applications in the fabrication of switches. MEMS offers advantages for these devices because it can combine excellent RF performance with circuit integrate ability and low power consumption. Recently MEMS technology has been applied successfully in the fabrication of another important RF device, the millimeter wave phase shifter. Researchers at the University of Michigan have fabricated a true-time delay phase shifter using surface micromachining on a quartz substrate with excellent insertion loss and phase-shift characteristics.
The present invention comprises a new type of transmission line structure that can be employed in MEMS-based phase shifters and switches, for example. The invention employs suspended transmission lines which are formed from spaced parallel electrically conductive beams that are laterally movable relative to one another using one or more microactuators. In the preferred embodiment, the beams are each formed from a single crystal silicon (SCS) core that is coated with metal, and the microactuators are comb-drive type actuators. Lateral movement of the beams by the microactuators to vary the spacing between the beams in a controllable manner enables the structure to act as a continuously variable phase shifter because the characteristic impedance of any section of the transmission line is a function of the beam spacing in that section. The same arrangement can be employed to move the beams of the transmission line into contact with one another, thereby acting as a switch.
Preferably, the transmission line includes first and second tunable capacitance sections that are separated by a third, matching section. The matching section is positioned at an angle, preferably a right angle, to the first and second tunable capacitance sections. The microactuators are connected to the beams at the corners formed between the first tunable capacitance section and the matching section and between the matching section and second tunable capacitance section so that the lateral spacing between the beams near the corners can be changed, and thereby the effective capacitance of the tunable capacitance section can also be changed. Because, for this (and other) transmission line topologies, the spacing between the lines is extremely small, the structure shows very little reflection from the discontinuities up to high frequencies. Thus, the variable capacitance sections at the corners induce variable phase shifts while the matching section functions to cancel the reflections from the first and the second elements. The unique aspects of this design for this application are that the geometry allows the matching section to maintain a constant spacing (characteristic impedance) throughout the actuation process, which simplifies both design and analysis. This results from the matching section running parallel to the actuation direction. The design also allows the beams to be bent much more easily, compared to standard fixedxe2x80x94fixed beams, because of the additional compliance of the bent beams that constitute the transmission lines. This means that at low voltages, the microactuators can provide a large tuning range and therefore a large phase shift. These unique qualities of the design result in a high-performance continuous microwave phase shifter on silicon, and has a number of advantages over other ways of making phase shifters: it is inherently low-cost, has low power-consumption requirements and the use of silicon as the substrate offers excellent thermal conductivity for heat sinking and enables the device to be integrated with other VLSI technology on chip.
In the switch application of the invention, the microactuators are employed to close selectively the two corner sections together completely. The matching section is then preferably chosen to reinforce the reflection from the two sections in the closed position around a certain design frequency. Moreover, the design enables full actuation of the device at relatively low voltages. This is again due to the fact that the bend in the middle of the transmission line beams makes them much more compliant.
In the fabrication of the preferred embodiment of the invention, bulk micromachining on high resistivity (p=2-3.5 kxcexa9-cm) silicon wafers is preferably employed. These structures are fabricated with the known SCREAM (Single Crystal Reactive Etching and Metallization) process, a low temperature bulk micromachining technology, which enables the fabrication of tall silicon beams suspended from the substrate. The large beam height, combined with a thick metallization ( greater than 1 xcexcm) yields transmission lines with very small ohmic losses. Since air is the only dielectric between the beams, dielectric losses and dispersion are minimized.